Telephone and electric power cable which extends between supports above the ground is commonly referred to as aerial cable. It may be extended between supports such as poles by stringing a metallic support wire in a catenary between two points and then lashing a multiconductor cable to the wire. In recent years there has been a trend toward the use of self-supporting aerial cables in which a supporting wire is run parallel with and adjacent to a cable core within a plastic jacket which encloses both the core and the support wire.
The advantages of a self-supporting cable construction should be readily apparent. It is only necessary to manufacture, ship and install a single cable having a common jacket rather than to ship and install a support wire and a separate cable which then must be attached to the support wire under field conditions.
The principal feature of self-supporting sheath cable is the incorporation of the stranded support wire which may be made of steel, for example, within the plastic outer jacket. Because of the weight of the cable in a catenary between two supports, the stranded wire undergoes some elongation during aerial placement of the cable. Moreover, because the outer jacket is common to the wire and because of its elastic properties, it undergoes the same extension. Provisions must be made for preventing transmission of the longitudinal strain, which accompanies such an extension, to the conductors to avoid undesirable capacitance effects while at the same time ensuring that the conductors are coextensive with the jacket so that they are accessible for splicing or terminating after the cable has been installed.
The above-mentioned problem has been overcome by the self-supporting sheath cable construction shown in U.S. Pat. No. 3,207,836 which issued on Sept. 21, 1965 in the name of H. C. Slechta. A core length which is somewhat in excess of sheath length is furnished by an undulation of the core prior to the application of a loosely fitted sheath. More specifically, a multiconductor cable core is set in an undulated form within an aluminum shield together with a stranded metallic support wire and both the support wire and the undulated core are enclosed in a common protective jacket. The undulations in the core permit the core to elongate within the metallic and plastic sheath without detrimental effects due to elongation of the support wire.
While the above-described cable design overcomes problems brought on by the method of support, aerial cable because of its exposure must have suitable provisions for substantially maintaining its integrity as a transmission means. Damage to aerial cable by squirrels and other sheath-chewing animals is a continuing problem. Due to unpredictable migrating patterns, outside telephone plant areas which had been thought to be safe from these animals are sometimes suddenly invaded, requiring the use of continuous protection along cable runs.
Various cable guards exist for protecting cables from damage by squirrels and other animals. One prior art aerial guard which is lashed to a cable is shown in U.S. Pat. No. 3,772,451. Another is self-locking and comprises a longitudinally elongated body having an inverted V cross-section with the legs of the V bent inwardly toward each other to define a substantially enclosed cavity for receiving and locking onto the aerial cable. See U.S. Pat. No. 4,159,395 which issued June 26, 1979 in the name of N. J. Cogelia.
The basic concept of a cable guard is to provide a barrier of relatively hard, slippery plastic around the cable which will prevent the animal from directly attacking the jacket of the cable so that any damage will be encountered by the guard and not the jacket. Also, the configuration of the guard and its slippery surface combine to eliminate a path of travel for animals along the cable.
Cable guards are most advantageous for applications in which operating companies are concerned with preventing additional damage to a cable which is already in use. The installation of cable guards necessitates an after-the-fact identification of problem areas, that is damage must have occurred before preventive measures are taken. Also, while guards provide means for preventing future jacket damage, their use requires more than an insignificant investment of capital.
As an alternative to the use of guards, thought has been given to a cable which itself includes provisions for withstanding attack by squirrels and for preventing the disruption of service. One cable design which has come into recent use and which is referred to as a reinforced aerial cable includes a core, comprising a plurality of polyethylene insulated conductors and being undulated as is shown in U.S. Pat. No. 3,207,836. Surrounding the core is a corrugated aluminum shield covered by a polyethylene inner jacket. A paper wrap is placed around the inner jacket to serve as a heat barrier during the subsequent soldering of a seam of a corrugated steel tape which is formed about the inner jacket. The inner jacket separates the aluminum shield from the steel shield thus preventing the possibility of moisture-originating corrosion between the two metals. The steel shield and a support wire that extends coextensively with the steel shield are enclosed in a common outer plastic jacket.
Problems which have been experienced in the field with cable of this so-called reinforced design include difficulties in the spiralling of the cable to inhibit wind-induced vibration which is prevalent in unobstructed areas. Another problem with this design has been the relative movement between the core and its covering shields and jackets such that it is not coextensive with the shields and the jackets. This requires that end portions of the covering shields and jackets be removed to gain access to the core.
These problems appear to derive from the tubular rigidity of the soldered steel member. Undulated core acts somewhat like a corrugated member in which the wave form can be compacted or extended. The priorly made, single shield, single jacket sheath is relatively compliant and tends to deform sufficiently during reeling to restrain the compacting of core, but readily permits the allowable extension. Apparently, however, the circularity which is maintained by the presence of the soldered steel in the reinforced design leaves the core free to compact as well as to expand. Inertial and gravitational forces which occur during cable unreeling and placement can be sufficient to cause the undulated core to settle or compact within the sheath.
What is needed is a self-support aerial cable which resists attack by animals and which overcomes the above-mentioned problems of spiralling and of core retraction. The prior art includes U.S. Pat. No. 3,638,306 which issued to H. N. Padowicz on Feb. 1, 1972 and which includes a core that is enclosed in an aluminum shield, having a longitudinal unsoldered seam and being coated with a waterproof material, in a steel layer having unsoldered overlapping edges, and in an outer jacket made of a plastic material.